1. Field of the Invention
The present invention relates to a bladder used to vulcanize a green tire for pushing the tire to open toward a mold by means of a pressurized heating medium and also to a tire producing method using such a bladder.
2. Description of the Related Art
One tire is produced by joining members including a carcass, beads, side walls, a belt and a tread into a green tire, setting the green tire in a mold, and applying heat and pressure to the green tire for a predetermined time to vulcanize it.
Upon vulcanization, a bladder 53 is placed inside a green tire 52 set in a mold 51 (see FIG. 4 which is a section showing a state where the bladder 53 is placed inside the green tire 52 set in the mold 51). By supplying a high-temperature and high-pressure heating medium to the inside of the bladder 53, the bladder 53 is expanded to press the green tire 52 against the inner wall surface of the mold 51 while opening the green tire 52 (see FIG. 5 which is a section showing a state where the green tire 52 is pressed against the mold 51 by the bladder 53). In this way, a tread pattern is engraved in the outer surface of the green tire 52, and the green tire 52 is vulcanized by being heated by the heating medium and the mold. A mixed fluid of steam and N2 gas of 180 to 200xc2x0 C. or the like is generally used as the heating medium.
The bladder needs to sufficiently expand and push the green tire from inside during vulcanization, whereas it is desired to be smaller than a spacing between the beads of the tire when the green tire is set in the mold and when the tire is taken out after being produced. Accordingly, bladders made of rubbers such as a butyl rubber having excellent flexibility have been used (prior art (1)).
However, the bladders need to have a thickness of about 4 to 6 mm in order to provide sufficient strength only by the rubber material. This results in a poor thermal conductivity, thereby presenting a problem of a longer vulcanization time.
Although the bladder should have such a shape (hereinafter, referred to as xe2x80x9cprofilexe2x80x9d) as to be closely in contact with the inner wall surface of the tire and uniformly press the tire during vulcanization, rubbers generally used as a material for the bladder possess a property of being cured in a high temperature environment. Thus, while the vulcanization of the tires is repeated, the bladder is cured to reduce the elongation rate of the bladder. As the elongation rate decreases, a pressing force by expansion is reduced, causing the profile to vary. If the profile is lightly variable, it does not cause any problem in the quality of the tires. However, in the case of the thick bladder as in the prior art (1), the variation of the profile becomes notable after the bladder is used a relatively small number of times. Thus, the bladder is obliged to be exchanged at an earlier stage (for example, the bladder is exchanged after vulcanization is repeated 300 to 400 tires in the case of automotive tires), which causes a problem of reduced productivity.
In view of the above, there has been proposed a thinner bladder by reinforcing the butyl rubber by tire cords made of aramid (prior art (2): U.S. Pat. No. 5,062,781). However, this bladder does not expand sufficiently since the tire cords hardly elongate. Thus, a satisfactory profile is difficult to form.
In addition, the bladder of prior art (2) has low shape flexibility and it is difficult for this bladder to constantly extend along the inner wall surface of the tire in a satisfactory manner. Thus, even if the bladder is formed with a gas escaping groove, gas remaining between the green tire and the bladder may not satisfactorily escape.
On the other hand, Japanese Unexamined Patent Publication No. 2000-317942 discloses a bladder made of a knitted or woven fabric from aramid fibers and formed beforehand into the same shape as the inner wall surface of a vulcanized tire (prior art (3)). This bladder is used as follows. First, the bladder is folded into a smaller piece and placed inside the green tire, and is expanded by the pressure of a heating medium to press the green tire during vulcanization. Since aramid fibers and the like are low extensible materials which are unlikely to change properties in a high-pressure environment, this bladder displays an effect that the profile of the inner wall surface of the tire is fairly stable even after vulcanization is repeated a large number of times. This Publication No. 2000-317942 also discloses a bladder made of an aramid-fiber fabric coated with a silicone resin or the like and having about the same air-tightness as the bladder made of butyl rubber (prior art (4)).
The present invention is directed to a bladder used to press the inner wall surface of a tire by means of a pressurized heating medium during the vulcanization of a green tire, comprising at least one rubber layer and at least one fabric layer, wherein elongation rates of the fabric layer in warp direction and in weft direction both exceed 15% and satisfy following equation (1):
0.4xe2x89xa6elongation rate in weft direction/elongation rate in warp direction xe2x89xa66.0xe2x80x83xe2x80x83(1).
The present invention is also directed to a tire producing method using the inventive bladder, comprising the steps of placing the inventive bladder inside a green tire set in a mold, and supplying a pressurized heating medium to the inside of the bladder to vulcanize the green tire while pressing it against the inner wall surface of the mold by the bladder.